There is a continuing need for tunable lasers and broadband optical amplifiers that operate in the near to mid infrared range for a variety of applications. These applications include optical fiber and free space communications, range finders, spectroscopy, chemical analysis, and LIDAR (Light Detection And Ranging). The spectral range of interest extends roughly from 1000 to 2000 nm, and devices in both bulk and waveguide formats are needed. While crystalline hosts doped with rare earth ions can be used to make optically pumped lasers with high power and efficiency, they are not tunable and do not satisfy the requirements for a wide gain-bandwidth. Rare earth ions in glass hosts have demonstrated excellent performance as optically pumped fiber lasers and amplifiers, with Er.sup.3+ at approximately 1540 nm being particularly successful. While their emission and absorption spectra are significantly broader than those of rare earth doped crystalline hosts, they are still narrow enough to severely restrict their spectral coverage. In addition, rare earth doped crystals and glasses provide optical gain at specific wavelengths that cannot be modified even if the host material is changed. For example, Er.sup.3+ doped glasses only provide useful gain from approximately 1520 nm to 1560 nm. Because almost all terrestrial optical fiber systems operate in the region of 1300 nm, it would be particularly useful to find dopant/host combinations that provide gain in a broad band centered at 1300 nm with efficiencies comparable to those of Er.sup.3+ doped glasses.
The two rare earth candidates for this role suffer from serious fundamental shortcomings. The Nd.sup.3+ emission spectrum is at a slightly longer wavelength than desired and suffers from severe excited state absorption on its short wavelength side. Significant gain has been obtained only for wavelengths greater than 1320 nm. Although Pr.sup.3+ provides gain at the desired wavelengths, the upper level of the gain transition is severely quenched by nonradiative relaxation even in fluorozirconate hosts, the best practical glass hosts available. This leads to high thresholds for lasers and large pump power requirements for amplifiers even when single-mode fibers are used.
Several transition-metal-doped crystals have been operated as optically pumped bulk lasers in the spectral range of interest. Examples include V.sup.2+, Ni.sup.2+, and Co.sup.2+ in a variety of hosts. More recently, laser action has been reported for Cr.sup.4+ in forsterite and YAG. Transition metal ions are generally characterized by extremely broad, vibronically assisted, optical transitions that can provide gain over an extended wavelength range. To date, however, only Co.sup.2+ :MgF.sub.2 has performed well enough to be available as a commercial laser in the spectral region beyond 1100 nm. There are no known reports of optical amplifiers in the 1300 nm spectral region based upon transition metal ions. Glasses are not practical as hosts for transition metal ions since they lead to a high nonradiative relaxation rate and thus a low quantum efficiency.
It is thus desirable to find materials that exhibit luminescence in the 1300 nm spectral region so as to be usable as optical amplifiers for fiber optic telecommunications applications. More generally, it is desireous to find a family of transition metal doped materials providing gain for use as lasers and optical amplifiers throughout the 1000 to 2000 nm range.